CIA security officers went to the aid of state department staff less than 25 minutes after they got the first call for help during the attack on the US consulate in Benghazi, Libya, intelligence officials have said.
The attack on the 11th anniversary of the September 11 terror attacks by what is now suspected to be a group of al Qaida-linked militants killed US ambassador Chris Stevens and three other Americans.
The detailed timeline was offered just days before Tuesday's presidential election in a clear effort to refute recent reports that said the CIA told its personnel to "stand down" rather than go to the consulate to help repel the attackers.
The officials told reporters that when the CIA annex received a call saying the consulate was under attack less than a mile away, about half a dozen members of a CIA security team tried to get heavy weapons and other assistance from the Libyans.
But when none was available, they went ahead with the rescue attempt. The officials said that at no point was the team told to wait.
The Obama administration's response to the attack on the consulate has been challenged by Republicans in Congress and elsewhere, questioning whether enough military and other support was requested and received.
It has become an issue in the election, with President Barack Obama's Republican challenger Mitt Romney and Republican politicians accusing the White House of misleading Americans about the nature of the attack.
Initial descriptions of the attack suggested that it may have been linked to a protest over an American-made anti-Muslim film.
Intelligence officials said they had early information that the attackers had ties to al Qaida-linked groups but did not make it public immediately because it was based on classified intelligence.